1. Field of the Invention
The present invention relates to a structure of a divided combustion chamber for a Diesel engine and, more particularly, to a structure of a divided combustion chamber of a Diesel engine, which structure is made of ceramics to have improved thermal efficiency and durability.
2. Description of the Prior Art
There have been made heretofore a number of trials in which a structure defining a divided combustion chamber, e.g., a swirl chamber of the Diesel engine is made of ceramics. Especially in order to improve the durability of the cup of the swirl chamber formed with an injection port, a variety of ceramics such as silicon carbide or silicon nitride have been tested because of their excellent heat resistances and high strengths. To improve a mileage or fuel consumption and a thermal efficiency, there have been made in recent years trials in which the swirl chamber described above is wholly made of ceramics so that the temperature of the combustion gas in the swirl chamber may be elevated by making use of the heat insulation of the ceramics.
Since ceramics are a brittle material, however, such a place, e.g., the aforementioned swirl chamber as is subject to remarkably severe thermal conditions is liable to be frequently ruptured by a thermal shock or stress even if the strength of the ceramics is raised to a high level by the recent technical progress.
When the structure forming the swirl chamber is wholly made of ceramics, an inner wall defining the swirl chamber is exposed to a high temperature for a very short time due to the combustion heat which is generated in the swirl chamber. On the contrary, the outer wall portion of ceramics forming the swirl chamber is held in close contact with the cylinder head or is in indirect contact with the same through air gaps, a heat insulating material or the like, so that its temperature is held low. This results in a remarkably large temperature difference in the direction of thickness of the ceramics forming the swirl chamber (i.e., the divided combustion chamber) so that a compressive stress is exerted upon the inner surface (i.e., that of the swirl chamber) whereas a tensile stress is exerted upon the outer surface.
Generally speaking, ceramics have a high compressive rupture strength but a low resistance to the tensile stress. As a result, when the aforementioned excessive tensile force is applied to the outer surface of the divided combustion chamber made of ceramics, cracks occur in a portion of the outer surface, and the ceramic begins to rupture. This is a major cause for reducing the durability of the divided combustion chamber.